Four parallel channels through the basal ganglia can be traced (Blumenfeld, 2002; Martin, 1996) each one targeting a different region of the frontal lobes. In PAEI order, these are the motor, prefrontal, oculomotor and limbic channels. Each one is discussed in more detail below.

P: Motor Channel
Cortical projections in this channel enter the basal ganglia primarily through the putamen, and leave via the internal segment of the globus pallidus (GPi) and the substantia nigra pars reticulata (SNr). Outputs project to the ventrolateral (VL) and ventral anterior (VA) nuclei of the thalamus. From there the channel ascends towards the premotor area (PMA), supplementary motor area (SMA) and primary motor cortex.

This channel is dedicated to the preparation for and control of action. Representation in the preSMA has been associated with the intention to act (Lau et al., 2004), the organization of action sequences (Kennerley et al., 2004), the preparation and execution of action (Cunnington et al., 2002), the endogenous generation of responses when environmental stimuli fail to provoke responses (Lau et al., 2004). The preSMA is thought to support cognitive motor control based on choices and discriminations made after stimuli have been received, whereas SMA-proper plays a main role in generating the readiness potential that precedes volitional, self-paced, voluntary movements. (Ikeda et al., 1999) The SMA seems to play a role in the suppression of sensation associated with voluntary action (Haggard & Whitford, 2004).

A: Prefrontal Channel
Cortical input to the head of the caudate leaves the basal ganglia via the GPi and SNr, projecting to the ventral anterior and mediodorsal (MD) thalamic nuclei, projecting to the prefrontal cortex (PFC) – locus of working memory, the conscious construction of representations, planning, prediction, extrapolation and evaluation. Specific NMDA receptors in the PFC has been shown to participate in the formation of contextual fear memories (Zhao et al., 2005).

The head of the caudate nucleus processes information about the fairness of a social partner’s decision, and the intention to trust that person once they have been deemed fair (King-Casas et al., 2005). The caudate is also central to ‘altruistic punishment’ – the desire to punish violations of social norms even when we have not been personally wronged (De Quervain et al., 2004). The head of the caudate is also implicated in obsessive-compulsive disorder and the regulation of ‘worry’ signals, in tandem with the orbitofrontal cortex (Whiteside et al., 2004; Remijnse et al., 2005).

E: Oculomotor Channel
Cortical input for this channel projects to the body of the caudate nucleus, and then to the VA and MD thalamic nuclei via the GPi and SNr. Output is directed towards frontal and prefrontal areas in the vicinity of the frontal eye fields. This channel is important for the higher-order control of eye movements and for spatial. The caudate is particularly implicated in the orientation of eyes towards rewards in the environment (Hikosaka et al., 2006) and for channeling spatial information. The body of the caudate is also implicated in the reward, motivation, and emotion systems associated with early-stage intense romantic love (Aron et al., 2005). It also plays a key role in classification learning; learning the relationships between stimuli and responses or cognitive categories (Seger & Cinotta, 2005). For these and other reasons, this channel thus seems to participate in (or partially overlap with) a reward-seeking or exploratory system.

I: Limbic Channel
Cortical input to this ventral channel arises from the temporal cortex, hippocampus and amygdala. Input enters the basal ganglia through the nucleus accumbens, ventral putamen and ventral caudate. Output to the thalamus emerges from the ventral pallidum, GPi and SNr, heading towards the MD and VA thalamic nuclei. These project to the anterior cingulate cortex and the orbitofrontal cortex – areas involved in the evaluation of personal actions and environmental resources, as well as social, behavioral and affective self-regulation.

This is a highly simplified and incomplete account of the brain regions described, but it serves as a starting point for understanding how the structure of concern may be embodied in the brain and in behavior.